Fossil fuels such as petroleum and coal are widely used in automobiles or power plants for generating motive force or electrical power. As known, burning fossil fuels produces waste gases and carbon oxide. The waste gases may pollute the air. In addition, carbon dioxide is considered to be a major cause of the enhanced greenhouse effect. It is estimated that the world's oils supply would be depleted in the next several decades. The oil depletion may lead to global economic crisis.
Consequently, there are growing demands on renewable energy. The common renewable energy includes for example solar energy, wind power, tide energy, terrestrial heat, biomass energy, and the like. Among various renewable energy sources, solar energy is expected to replace fossil fuel as a new energy source because it provides clean energy without depletion. As a consequence, the solar energy is gradually adopted for establishing a distributed power supply system.
Although the solar energy has broad scope of application, there are still some drawbacks. For example, the solar generator is usually used for the household customers. Since the input voltage of the DC-to-AC converting circuit (or photovoltaic inverter) used in the solar generator is dependent on the magnitude of the input voltage generated by the solar cell, the output voltage of the photovoltaic inverter is decreased as the intensity of the solar beam becomes weak.
FIG. 1 is a schematic circuit diagram illustrating a photovoltaic inverter according to the prior art. As shown in FIG. 1, the photovoltaic inverter 10 includes a front-end boost circuit 101, an intermediate DC buck circuit 102 and a rear-end DC-to-AC converting circuit 103. By the front-end boost circuit 101, the input voltage Vin generated from a solar cell 2 is converted into a bus voltage Vbus, which is slightly influenced by the solar beam intensity and has a high DC voltage value (e.g. 400V). By the intermediate DC buck circuit 102, the bus voltage Vbus is decreased to a first DC voltage V1 (e.g. 220V). By the rear-end DC-to-AC converting circuit 103, the first DC voltage V1 is converted into an AC output voltage Vo. In a case that the intensity of the solar beam is weak and the input voltage Vin is too low, the front-end boost circuit 101 may maintain a constant voltage value of the bus voltage Vbus. Correspondingly, the magnitude of the AC output voltage Vo is not influenced by the intensity of the solar beam.
The photovoltaic inverter 10 is a three-stage converting circuit. As known, the use of front-end boost circuit 101 leads to inferior operating efficiency of the photovoltaic inverter 10. In addition, the photovoltaic inverter 10 is not cost-effective.
Therefore, there is a need of providing a DC-to-AC converting circuit with a wide input voltage range so as to obviate the drawbacks encountered from the prior art.